The ads posted here aren't designed to sell products directly. It's all to do with how Google indexes pages.
Google's PageRank algorithm uses incoming links as a measure of importance for a page. So spammers will aim to get their site linked to from as many other places on the Internet as possible. Dumping long lists of keywords linked back to their website on forums, blogs, etc. means that they'll move up the Google index. (The further up the front page of search results for a particular keyword, the more hits - and so sales - the website will get).
The posting is typically done by automated programs which are designed to seek out forums, create accounts, and post messages. The spammer typically just has to enter their message and press a button to set the robot going.
There are moves to combat this by requiring the user to enter a unique code when posting (typically copied from a distorted image that a computer program can't read, but a human brain's sophisticated visual processing system can), but as fast as countermeasures are created then the spammers find a way around them.
Spam is incredibly profitable as the overheads are tiny. The average bulk mail letter sent via the Postal Service may cost a dollar or two, whereas sending out an e-mail costs millionths of a penny.
Spam campaigns can remain financially profitable even if only one in ten million recipients takes up the offer (and screw the other 9.9 million people who get annoyed by the message), so yes, there is unfortunately a strong incentive to spam.